This invention relates to systems for manufacturing nuclear fuel green pellets and particularly to such systems wherein the process is automated.
In many designs of nuclear reactors, the reactor vessel has an inlet and outlet for circulation of a coolant in heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound, is stacked within the metallic sheath. During reactor operation, the nuclear fuel pellets decompose releasing fission products such as fission gas while generating heat in a manner well known in the art.
There are many known methods for manufacturing the nuclear fuel pellets used in nuclear reactors. Most of these methods generally consist of cold pressing a powder which may be an oxide of fissionable material such as uranium dioxide to form dense compacts. These dense compacts are generally referred to as green pellets. The green pellets are then sintered in a non-oxidizing atmosphere to produce a sintered pellet which may have slight irregularities on its surface. The sintered pellet may then be ground to remove those irregularities thereby forming a right cylindrical pellet. This finished pellet is then stacked within the metallic sheath to form the fuel element that may be used in a nuclear reactor.
A commonly known method for producing the nuclear fuel pellets is described in U.S. Pat. No. 2,991,601 to J. Glatter et al, issued July 11, 1961. In this process, hydrogen reduction of uranium trioxide is employed to produce uranium dioxide powder. As received from commercial manufacturers, this uranium dioxide is not free flowing and is, therefore, not adaptable for use in automatic machinery for the production of the green pellets. In order to produce a free flowing powder, the uranium dioxide powder is mixed with a suitable binder such as aluminum stearate and water to form a wet granulate. The wet granulate is then forced through a screen and dried, after which it is dry-screened thereby separating the larger particles from the smaller particles. The water may be substantially removed in the later sintering process while the aluminum stearate will remain and act as a lubricant in the compacting process. Once the uranium dioxide powder has thus been converted into a free flowing granulate, the granulate is then compacted into green pellets in a cold pressing operation. The compacting process comprises flowing the granulate into a die and cold pressing the granulate in the die into substantially cylindrical green pellets. The green pellets may then be heat treated, sintered and ground to form the finished pellet for use in nuclear fuel elements.
While the patent to Glatter and other known methods illustrate commonly understood methods of manufacturing green pellets, these methods all involve relatively small volume production. Because the prior art methods involved small volume production, these processes were performed in a glove box environment. Each process was performed in a separate glove box type enclosure and then moved under secure conditions to the next glove box where the succeeding step was performed. This glove box arrangement not only required long time intervals during bag-out transfer between glove boxes, but it also required a large amount of floor space to accommodate the glove boxes. Furthermore, the glove box enclosure did not provide adequate accessibility to the apparatus therein due to the limited capability of the typical glove box arrangements. With the demand for nuclear fuel increasing it became a commercial necessity to be able to mass produce the green pellets. Such mass production would entail larger apparatus and faster throughput both of which were not compatible with conventional glove box arrangements. In addition, the recent use of plutonium dioxide in a mixed oxide pellet increased the safeguards which must be employed to assure accountability of the plutonium. The necessity of moving the plutonium from one glove box to the next in a large volume process would create serious accountability problems that result in substantial time delays during such transfer between glove boxes. All of these problems together render impractical a large volume production of a mixed oxide green pellet in a typical glove box arrangement.